Dynamic Range Control is an audio signal processing technique that narrows or “compresses” the swing or excursion of an audio signal, so that loud portions of the signal are made quieter while quiet portions may be made louder (if desired). DRC may also expand the signal swing or excursion. The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. The DRC may be done to improve audibility in noisy environments. For example, increasing the volume to better hear the quiet portions in a noisy environment could make the loud portions too loud. Compression reduces the level of the loud portions, but not the quiet portions, so that volume can be raised to a point where the quiet portions are more audible without the loud portions sounding too loud. A compressor applies negative gain to an audio signal if its amplitude (in a given portion) exceeds a certain threshold to perform downward compression. Typically, an input/output ratio is defined that determines the amount of compression. For example, a 4:1 ratio means that an input signal overshooting a threshold by 4 dB will leave the compressor 1 dB above the threshold, i.e. a gain of −3 dB has been applied by the compressor to that portion of the signal. Also, in practice, a compressor exhibits a delay before its output level is actually reduced to the required level—this is referred to as the attack phase.
Somewhat similar to a compressor, a limiter also limits loud sounds. However, it does so in a much more abrupt manner, in effect exhibiting a much higher ratio and a much shorter attack phase. Limiting is typically used as a safety device rather than as a sound-sculpting tool.